Pyroelectricity is a property of certain crystals consisting of polarization developed in a crystal by an inequality of temperature. Spontaneous or stress-induced polarization, signifyig pyroelectricity and piezoelectricity respectively, can appear in ionic solids solely due to a non-centrosymmetrical spatial distribution of ions in a polar crystalline structure. Although theory does not impose strict limitations on the size of a polar crystallite[1,2], the magnitude of pyroelectric and piezoelectric effects of some ceramics, particularly BaTiO3, rapidly decrease as grain size diminishes to a few nanometers[1,3,4].
Determination of the minimal number of periodically arranged unit cells for which a crystal retains pyroelectric and piezoelectric properties has become increasingly important due to the rapid incorporation of these materials into nanometer-scale devices.
As disclosed in U.S. Pat. No. 5,504,330, pyroelectric properties of a thin film made of perovskite materials may be enhanced, by the addition of lead to an original perovskite material having an original ferroelectric critical grain size, and then forming a layer of the lead enhanced perovskite material having an average grain size less than the original ferroelectric critical grain size. The remanent polarization of the layer appeared to be substantially greater than the remanent polarization of the original perovskite material.
Thin BaTiO3 ferroelectric films are important for a number of applications such as high charge density capacitors, ferroelectric memory, and microwave and optoelectronic devices. However, integration of BaTiO3 into Si microfabrication technology is hindered by the high chemical reactivity of BaTiO3 with respect to Si. Therefore, despite the small misfit between the lattice parameter of Si and inter-plane distance of BaTiO3, epitaxial growth of BaTiO3 on Si always requires intermediate buffer layers.
DE 10028022 discloses the production of highly ordered low molecular inorganic thin action layers at low temperature on silicon chips in pyroelectric detectors. Such production comprises applying an action layer made from a ferroelectric, pyroelectric or piezoelectric material to a substrate with a highly ordered polytetrafluoroethylene coating whereby the action layer is ordered through the polytetrafluoroethylene substrate.
It is also known that pyro- and piezo-electric effects may exist in structures that lack the spatial periodicity inherent for ionic crystals, but composed of polar molecules with directional ordering. An example of such a material is a nematic liquid crystal. Spontaneous or stress induced dipole ordering without fine-tuned positional order is theoretically possible in ionic solids[5] as well; however only indirect experimental evidence supporting this theory has been presented so far[6-9].